Muscle & peripheral nerve nontumor
Inflammatory myopathies
Polymyositis
Author: Meggen Walsh, D.O., M.S., P.A.
Topic Completed: 1 June 2016
Minor changes: 12 January 2021
Copyright: 2002-2021, PathologyOutlines.com, Inc.
PubMed Search: Muscle [title] + Polymyositis [title]
Table of Contents
Definition / general | Essential features | Terminology | Epidemiology | Sites | Pathophysiology | Etiology | Clinical features | Diagnosis | Laboratory | Radiology description | Prognostic factors | Case reports | Treatment | Gross description | Microscopic (histologic) description | Microscopic (histologic) images | Cytology description | Positive stains | Negative stains | Differential diagnosis | Additional referencesCite this page: Walsh M. Polymyositis. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/musclepolymyositis.html. Accessed May 15th, 2021.
Definition / general
- Inflammatory myopathy that targets predominantly skeletal muscle
- Can be seen in other autoimmune diseases
Essential features
- Skeletal muscle shows myopathic changes with evidence of myofiber degeneration and regeneration with lymphocytes invading non necrotic myofibers
- Predominant inflammatory cell is the CD8+ T lymphocyte
Terminology
- Polymyositis or idiopathic polymyositis
Epidemiology
- Prevalence worldwide is 5 - 21.5 per 100,000 (Ann Rheum Dis 2009;68:1192), but only 3.8 per 100,000 in US (Muscle Nerve 2012;45:676)
- Peak incidence is in older patients, with a mean age of 56 years and a male to female ratio of 1.6 : 1
- Extremely rare in younger patients and childhood
Sites
- Proximal limb muscles are commonly affected
- Dysphagia in 30% of patients
- Heart muscle and extraocular muscles are generally spared (Yachnis, A. and Rivera-Zengotita, M., High Yield Pathology: Neuropathology, 1st ed., 2012)
Pathophysiology
- MHC1 restricted CD8+ T lymphocytes are antigen driven to destroy myocytes via perforin (Curr Rheumatol Rep 2009;11:287)
Etiology
- Not fully known
- In a large study from Europe, Anti Jo antibodies were seen in 22% of polymyositis patients (Ann Rheum Dis 2015;74:1551)
- Other antibodies are anti PL7, anti PL12, anti signal recognitions particle (SRP), anti Mi2
- Genetics may play a role, since more likely in certain HLAs: HLA A*01, Cw*07, DQA1*0501, DRB1*0201, C4A*Q0, DRB1, DQA1*0501, DQA1*0401, B7, DRw6, and DRB1*0803
Clinical features
- Most common reported feature is muscle weakness (97%), typically proximal symmetric weakness that progresses slowly
- Myalgia, dysphagia and dyspnea can be seen
- Raynaud's is also be seen but is more common in dermatomyositis
Diagnosis
- Clinicopathologic diagnosis
- Histology shows myopathic features with inflammation; however, review of drugs and clinical history is required
Laboratory
- Serum CK (creatine kinase) and aldolase are elevated in 90%
- Elevated sedimentation rate (ESR) of 50 - 100 in 40% and an ESR above 100 in 10% (Ann Rheum Dis 1993;52:857)
Radiology description
- MRI T2 may show edema, which is useful in muscle biopsies
Prognostic factors
- Response to therapy is variable - older patients at diagnosis do worse
- In one study, the 10 year survival rate was 50%; the main cause of death was circulatory followed by "musculoskeletal" (Clin Rheumatol 2006;25:234)
- Dysphagia and recalcitrance to therapy are poor prognostic factors
Case reports
- Child presenting with polymyositis from graft vs host disease (Arch Neurol 1982;39:188)
- 16 year old with anti Ku Antibody positive polymyositis (Pediatr Dermatol 2015;32:e224)
- 48 year old female with polymyositis anti Jo antibody induced by entanercept (Clin Rheumatol 2010;29:563)
- 70 year old male with bladder cancer treated with pelvic radiation leading to polymyositis and fatal interstitial pneumonia (Clin Case Rep 2015;3:710)
- Polymyositis manifesting two years after diagnosis of unilateral temporal myositis (Med Ultrason 2016;18:123)
Treatment
- Steroids - most patients respond to just corticosteroids
- Immunomodulators (rituximab, infliximab) and immunosuppressive agents (cyclophosphamide, cyclosporine) have been used and patients have seen improvement (Eur Neurol 2003;50:10, Rheumatology (Oxford) 2011;50:2155, Neuromuscular Disord 2004;14:337)
- Recovery may take time and may not be complete in some patients
Gross description
- Skeletal muscle gross findings are non specific
Microscopic (histologic) description
- Skeletal muscle shows myopathic features - occasional small rounded myofibers with mildly increased internal nuclei
- Individual myofibers show degeneration with possible macrophage infiltration to clear the necrotic myofiber
- May be myofiber regeneration with basophilic myofibers with large nuclei and prominent nucleoli
- Should be a prominent endomysial inflammatory infiltrate (this can be attenuated if the biopsy is after steroids/therapy)
- Lymphocytes invading non necrotic myofibers are a diagnostic finding
- Occasional cases show only lymphocytes associated with necrotic myofibers and the possibility of polymyositis can be suggested if clinically applicable
- Biopsy can also show mild to moderate fibrosis
Microscopic (histologic) images
Contributed by Meggen Walsh, D.O., M.S., P.A.
Various images
Cytology description
- Of little to no benefit, could see non specific lymphocytes
Positive stains
Negative stains
- Dystrophy panel is normal
- No loss of enzyme histochemical stains
- Gomori trichrome shows no rimmed vacuoles
- No vacuoles identified by LC3 or ubiquitin
Differential diagnosis
- Immune Mediated Necrotizing Myopathy: can occur post-statins or de novo; myopathic features, necrosis and myofiber regeneration, but no inflammatory infiltrate
- Inclusion body myositis:
- Similar inflammatory pattern
- Rimmed vacuoles may be difficult to find or not be "classic"
- Presence of occasional granular staining suggestive for vacuoles using LC3 or ubiquitin requires clinical correlation to determine whether these features fit best with PM or IBM;
- Some cases are seen with positive anti-SRP (signal recognition particles) (Ann Rheum Dis 2006;65:1635)
- Statin therapy and post-statin therapy: can appear similar to the findings of PM: thus clinical and laboratory correlation may be required
